// 判断大鱼和果实的距离
function momFruitsCollision() {
  for (let i = 0; i < fruit.num; i++) {
    if (fruit.alive[i]) {
      var l = calLength2(fruit.x[i], fruit.y[i], mom.x, mom.y);
      if (l < 900) {
        fruit.dead(i);
      }
    }
  }
}